Skeletal structures Counting total electrons Compound 4) Octet for O Cl O peripheral 16 - 12 = 4 atoms 5) Remain1) Skeletal O Cl O O Cl O ing e–s on Structure center atom 4 - 4 = 0 2) Count 7x1 + 6x2 6) Create electrons multiple +1 = 20 No need bonds? 3) Electron O Cl O Final O Cl O pairs in structure or 20 - 4 = 16 O Cl O bonds ClO2– Resonance structures • Lewis structures for certain atoms do not match experimental observations • For example, the bond lengths of CHO2– predicted by the Lewis structure are incorrect – O H C O • The double CO bond should be shorter, and possess a greater bond energy (due to the higher concentration of e–s in a double bond) • Yet, experimentally, both bonds are the same • The reason is due to “resonance” [ ] Compound 4) Octet for C O peripheral atoms 8-6=2 5) Remain1) Skeletal C O C O ing e–s on Structure center atom 2 - 2 = 0 2) Count 4x1 + 6x1 6) Create electrons multiple C O = 10 bonds? 3) Electron Final C O C O pairs in structure or 10 - 2 = 8 bonds C O • Try PE7 (pg. 241) (answer for HClO4 is wrong in book - missing an O) CO Resonance structures • A resonance structure can be drawn for any molecule in which a double bond can be formed from two or more identical choices • Resonance structures can be drawn 2 ways… – – O 1 H C H C O – O 2 H C [ ] [ [ O Placing electrons around atoms Lewis Structures • Once we have determined the number of total valence electrons we can start distributing them throughout the molecule • The rules for this are outlined in fig. 7.4 (pg. 241). These also appear on study notes (with additional details added) • When we represent electrons they will be in pairs (since an orbital holds 2 electrons) • Electron pairs can be represented with 2 dots or a solid line … O • Once we have determined the basic structure of the molecule we can start placing electrons around atoms • The first step is to determine the total number of electrons that are available • We use the group number of an element to indicate the number of valence electrons that it contributes to the molecule. • E.g. O in group VIA (6A), contributes 6 e–s • Read 7.6 (pg. 237) up to PE6 (including example). Do PE6. O 2 peripheral Lewis’ bonded to a central Lewis • Because there are exceptions to the octet rule, we need a set of rules to determine how many electrons surround atoms • The first step is to determine how the atoms are bonded in a molecule • Generally, if there is only one of one element and multiple copies of another element, the unique element is central • Commonly, H is peripheral, bonded to O • Read 7.6 (pg. 236) up to PE5. Do PE5. ] ] • Resonance implies that the bond flips back and forth. Really, it lies between extremes • Read 7.7 (pg. 242). Do PE 8.